1. Field of the Invention
The present invention relates in general to gas scrubbers, and in particular to an improvement for center inlet type scrubbers.
2. Description of the Related Art
Several types of gas scrubbers are known in the FGD (flue gas deulfurization) industry. A detailed discussion of FGD construction and use can be found in STEAM , 40th Edition, Babcock & Wilcox Company, Chapter 35. Center inlet type scrubbers have a unique design as shown in FIG. 1. The scrubber shown in FIG. 1 consists of two concentric structures. The outer structure contains spray headers 12 delivering the scrubbing solution and may contain one or two stages of mist elimination devices 30. In some cases, one or both stages of the mist elimination device 30, are located in the outlet flue 14. The inner center structure consists of the inlet flue 16, a venturi section 18 used to quench the gas and separate ash particles from the gas stream, and a downcomer 20 to deliver the gas downwardly under the level of the sprays. A plumb bob 22 is used to control and accelerate the entering gas to insure maximum particulate separation.
The entering gas is quenched in the venturi section 18 located above the downcomer. The gas flow rate and efficiency of particulate separation is controlled by the plumb bob position and the pressure drop across the plumb bob 22. In the inlet downcomer 20, the cleaned and quenched gas travels downwardly with the quenching/scrubbing solution. At the bottom of the downcomer 28, the quenching/ash collecting solution drops into an integral tank 24 or a device directing this stream into a separate tank. The gas makes a 180.degree. turn and travels upwardly in a countercurrent direction towards the flow of the spray headers 12. In the outer annulus 26 of the scrubber, several levels of sprays are located midway between the bottom 28 of the downcomer and the absorber outlet 14. Each spray header contains one or more nozzles that sprays the scrubbing solution or slurry which is normally alkaline counter to the gas flow. The scrubbed gas exits through the scrubber outlet 14 located at one side of the outer annulus 26. Mist is removed from the cleaned gas in one or more stages of mist elimination device 30.
The mist elimination device(s) may be located horizontally in the vertical section of the outer annulus 26, or may be located vertically in the scrubber outlet 14. The configuration of the scrubber, the 180.degree. turn at the bottom of the downcomer, and the location of the gas outlet promote gas maldistribution in the outer annulus 26. A typical gas velocity profile for this type of scrubber is shown in FIG. 2 with gas velocities being in feet per second.
The main disadvantage of the prior art system is the poor gas distribution in the spray section. The entering gas leaves the bottom 28 of the downcomer at a velocity in excess of 50 to 90 feet per second making an unguided 180.degree. turn. The combined effect of the high gas velocity, the turn, the location of the outlet, and the lack of guiding devices, such as gas turning vanes, at the bottom of the outer annulus forces the gas towards the outer walls of the annulus and in the direction of the outlet. Poor gas distribution leads to excessive localized velocity zones near the outer walls, gas recirculation, poor gas liquid contact, and poor removal efficiency for a given scrubbing liquid flow rate. The poor gas distribution and the localized high velocity pockets are passed on to the mist elimination devices leading to failure in their performance and/or uneven solids deposits on the surface of the mist eliminator 30. This uneven flow through the mist eliminator 30, leads to pluggage of the low liquid flow zones which leads to even higher gas velocity, and increased potential for faster failure and liquid droplets carryover to the stack.
The addition of turning vanes or other gas distribution devices in the outer annulus at the bottom of the downcomer, such as guiding, or turning vanes may correct the gas distribution problem, however, the use of scrubbing slurry and contamination of the slurry with the removed ash presents a potential for partial solid build-up on the vane and eventual total pluggage of the gas flow area. Such a solution to this problem is viable only if the scrubbing solution is clear, and the products of the scrubbing process are highly soluble in the scrubbing liquor to prevent solids deposition.
The use of pie shaped trays to correct gas maldistribution has the potential to aggravate the problem because the pie shaped tray sections are wide near the outside of the annulus where gas flow area need to be restricted, and narrow near the downcomer where a bigger flow area is necessary to promote greater gas flow. Also, each pie shaped tray section requires two supports spanning the radius of the annulus from the outer wall to the downcomer. The proximity of the supports near the downcomer also contribute to the restriction of the flow area near the downcomer.
Another option is employed in the present invention. This is to place a perforated tray at the bottom of the downcomer to affect gas distribution as the gas turns at the bottom of the annulus, and improve pollutants removal by the following methods:
Providing even and optimum gas/liquid proportions at and above the perforated tray; PA1 Allowing longer contact time between the gas and the liquid, because the perforated tray acts as a contacting surface, as well as a gas distribution device; PA1 Acting as a storage bin for the liquid draining from the tray to the tank underneath, therefore, promoting cleaning of the structures under the tray; PA1 Straightening the gas flow upstream of the spray banks and providing maximum utilization of the scrubbing reagent; PA1 Locating the perforated tray at a proper distance from the lowest spray bank to avoid impingement of the spray on the tray and minimize erosion of the tray material; PA1 Locating the perforated tray at a proper distance from the lowest spray bank to insure that the spray is well developed before it is interrupted by any structures; and PA1 Building the tray sections of rectangular or non pie shaped geometric structures to maximize the flow area near the downcomer and minimize the tray section supports.